1. Field of the Invention
The present invention relates to a power transmission device. More particularly, the invention relates to a power transmission device for a clutchless compressor, in which, when a torque above a preset value is generated in the compressor during power transmission from an engine to the compressor, a breaking member connecting a pulley side with a hub side is broken and also the breaking member is released from the structure, thereby preventing a failure of engine belt and unnecessary noise.
2. Background of the Related Art
In general, a car air-conditioning system serves to maintain the interior temperature of a car below an outside temperature through a cyclic operation of compression, condensation, expansion and evaporation of refrigerant.
The above cyclic operation requires essentially a compressor, a condenser, an expansion valve and an evaporator.
Among the above-required devices, the compressor includes a variable displacement compressor, in which a required power can be controlled, depending on the conditions of the system. Recently, this variable displacement compressor has widely used.
This variable displacement compressor does not need a clutch for interrupting a driving power transmitted from an engine to the compressor.
During the operation of the compressor, occasionally, an overload torque way above a normal transmission torque occurs inside the compressor, due to a failure such as seizing, so that the pulley can not rotate any more.
At this time, the engine belt, which is driven by the engine, keeps sliding on the pulley and wearing itself (a belt slip), and consequently is broken due to the friction heat generated between the pulley and the belt.
As one attempt in order to solve the above problems, Japanese Laid-open Patent Application Ser. No. 1998-299855 discloses a power transmission device, which is shown in FIG. 1.
As shown in FIG. 1, the convention power transmission device 1 includes a coupling means. The coupling means includes a pulley 2 to be rotated by a rotating power from a rotation power source, a hub 3, 4 coupled to a rotation shaft of a driven-side device, an resilient member 5 connecting the hub 3, 4 and the pulley 2 and being capable of elastic deformation, and a first and second support member 6, 8 for supporting the resilient member 5.
The resilient member 5 is installed concentrically with the hub 3, 4 and the pulley 2. The first support member 6 is connected with the hub 3, 4 and the second support member 8 is connected with the pulley 2.
The first support member 6 supports one side face of the outer circumference or the inner circumference of the resilient member 5. The second support member 8 supports the outer peripheral face of the outer circumference and the inner circumference of the resilient member.
When the rotating force is within a certain predetermined value, the coupling means presses in the rotational direction the resilient member 5 and the both support members 6, 8. Thus, the resilient member 5 is integrally supported between both of the support members 6 and 8 such that the hub 3, 4 is connected integrally with the pulley 2.
When the rotating force is increased above the predetermined value, i.e., under overload, at least one of the outer circumference and the inner circumference is deformed and thus a sliding is occurred between the surface of the resilient member 5 and at least one of both support members 6 and 8. Therefore, the connection between the hub 3, 4 and the pulley 2 is interrupted.
In addition, a low rigid member 7 is formed in at least one side of the both support members 6 and 8. In the region of a higher rotating power, at least one side of the both support members 6 and 8 is deformed by the low rigid member 7, thereby lowering a relative torsional spring coefficient between both support members 6 and 8 against the rotating power.
The assembling procedures of the conventional power transmission device having the above-described construction will be explained below.
First, the second support member 8 is installed in the pulley 2, which is then installed in a nose portion 51a of a front housing 51 in a compressor 50.
Thereafter, the resilient member 5 is adhered and fixed on the second support member 8.
Next, an installation body, which is comprised of the hub 3, 4 and the first support member 6 integrated therewith, is assembled into the resilient member and the rotating shaft of the compressor. This installation body is fixed to the rotating shaft using a bolt 9 to thereby finish the assembling.
As described above, the conventional power transmission device having the above construction is comprised of a number of elements such as a pulley 2, a hub 3, 4, a first and second support member 6, 8, and a resilient member 5, which leads to complicated assembling procedures and consequently a long assembling time.
In addition, the convention power transmission device 1 embraces a further problem in that the manufacturing cost is increased, due to such a large number of constitutional elements.
Furthermore, in the conventional power transmission device 1, the pulley is formed of a ferrous metallic material or a plastic material using an injection molding process. Depending on the material of the pulley 2, the forming method for the resilient member 5 and the first and second support member 6, 8 is varied and thus a common forming process cannot be employed disadvantageously.
In addition, in the conventional power transmission device 1, if the design specification of the pulley 2 is changed, those of the resilient member 5 and the first and second support member 6, 8 must be changed together.
On the other hand, FIG. 2 shows another conventional power transmission device. As depicted in FIG. 2, this conventional power transmission device includes a pulley 10 rotatably installed in a front housing 51 of a compressor and rotated by a rotating power from a driving power source such as an engine, a hub 20 to be coupled to a driving shaft 60 of the compressor, and an outer hub 30 connecting the pulley 10 with the hub 20.
FIG. 3 shows a hub in the conventional power transmission device of FIG. 2. As shown in FIG. 3, the hub 20 includes a hub body 21 to be connected to a driving shaft 60 of a compressor 50, an inner ring 22 formed in the outer peripheral face of the hub body 21, an outer ring 23 arranged spaced apart from the inner ring 22 and having plural through-holes 23a so as to increase the connection force with the outer hub 30, and a breaking member 24 connecting the inner ring 22 and the outer ring 23.
The outer hub 30 is made of resin. The inner circumference of the outer hub 30 is connected to the outer ring 23 of the hub 20 using an insert injection molding. The outer circumference thereof is coupled to the pulley 10 by inserting a pin 31 into a damper 32, which is provided in a hole 11 formed in the pulley 10. Therefore, power transmission can be carried out from the pulley 10 to the hub 20.
Therefor, in case where the compressor-side load torque is within a pre-setup value, the rotating power of the pulley 10, which is connected with an engine through an engine belt, is transmitted to the driving shaft 60 of the compressor 50 through the outer hub 30 and the hub 20, thereby driving the compressor 50.
On the other hand, when the compressor 50 produces a torque above the pre-determined value, the driving shaft 60 and the hub 20, the pulley 10 and the like stop rotating. At this time, the engine and the pulley 10 connected thereto by the engine belt keep trying to rotate, and consequently the breaking member 24 of the hub 20 is broken, thereby instantly interrupting the power transmission path form the pulley 20 to the driving shaft 60 to thereby avoid damage of the compressor.
That is, when a torque above a preset value is produced in the compressor 50, the breaking member 24 of the hub 20 is broken to thereby protect the compressor from damage. At this time, however, the pulley 10 keeps rotating and the hub 20 stops. Thus, the broken side of the breaking member 24 makes excessive noise.
In addition, if the interference of the broken side continues, impact load is exerted to the engine belt several times, which also consequently is failed.
Furthermore, since it has no structure for fixing the outer ring 23 of the hub 20, the outer hub 30 can be released from the transmission device, together with the outer ring 23, after the breaking member 24 is broken. In the case where the outer ring 23 and the outer hub 30 are escaped, they hit other peripheral equipment, which thereby suffers a fatal damage.